There often exists a need for regulating the fluid pressure and/or the fluid flow in fluid distribution networks. An example of such a fluid distribution network is a water distribution system used in cities for distributing drinking water from a central reservoir to a plurality of households and other water consumers. Such networks or systems typically comprise one or several main water lines or conduits that convey water from a reservoir or a pump station to different zones of the network. Each zone normally comprises a secondary water line or conduit which conveys water to different areas in the zone. In each such area a consumer distribution line supplies drinking water from the secondary conduit to each individual consumer within the area. Typically the pressure in the main conduits is around 10 bar, whereas the pressure in the individual consumer lines is kept at around 3 bar. Therefore the distribution network often comprises pressure reducing means which normally are arranged at the junctions between the main and secondary conduits and/or between the secondary conduits and the consumer lines. The pressure reducing means may however also be arranged within the main, secondary or the consumer line conduits. In other systems there may be arranged additional intermediate water line conduits operating at different pressures between the main water line and the consumer distribution lines. Traditionally the pressure reducing means are constituted by conventional pressure valves, which often are spring loaded or controlled by a pilot pressure. Another widely used method is to arrange break pressure tanks in the pipeline, to break pressure down to atmospheric pressure before gravity builds pressure in the downstream pipeline.
In order to be able to utilize the energy of the fluid, it has been suggested to provide the pressure reducing valve in such a water distributing network with a turbine which is arranged to capture energy which is released during pressure reduction. U.S. Pat. No. 6,824,347 B2 discloses such valve.
WO 2008/004880 discloses an apparatus that may be used as a turbine for reducing pressure in water distribution networks and efficiently producing electricity from the pressure difference when connected to a generator. Another benefit of the apparatus of WO 2008/004880 is that it is capable of controlling the flow and the downstream pressure. In other types of water turbines the downstream pressure is either atmospheric, or they cannot control flow or pressure (e.g. propeller-based turbines).
The apparatus of WO 2008/004880 has two impellers arranged on two parallel shafts. When it has been installed in water distribution networks, gears have been used for transferring the force between the two shafts and one of the shafts has been coupled to a generator connected to the power grid. Another function of the gears has been to synchronize the rotational movements of the two impellers. Thus, such gears are sometimes referred to as timing gears.
In case of a failure in the above installation, the impeller may begin to spin freely (“free spinning”). The consequence can be pressure bursts and surges, which can be damaging for the pipeline system. To handle the free spinning incidents, water bypass arrangements including pressure reduction valves have been installed. However, such bypass arrangements are expensive and require space, which often is limited in e.g. municipal water systems.